I'm no contract guru, but there are two forms of reroute in the definitions section of section 23. One is "1) delete a previously scheduled flight segment(s), and/or 2) add a flight segment(s) that is not open time (including flying removed from open time)," and the other is doing that with flight segments from open time using the 23.N or 23.O open time ladder.

The first one doesn't require the segments to work through the open time ladder, so I think groundstop would be wrong. Skeds can just reroute you with any segment they want, as long as it works. The second form of reroute uses the open time ladder and segments/rotations have to follow the assignment order.

Obviously, when things are busy they'll just pull the rotations from open time and reroute pilots on the fly. Which bring us to our problem of pilots on long sits turning into defacto airport reserves (obviously you don't have to answer the phone while roaming the concourses, but they'll just reroute you via ACARS on the way into ATL or DTW, etc).